Long-Term Evolution (LTE) is the next step in cellular Third-Generation (3G) systems, which represents basically an evolution of present mobile communications standards, such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM) [1]. It is a Third Generation Partnership Project (3GPP) standard that provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink. It uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations. LTE is also expected to improve spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth.
One of the growing fields of interest in 3GPP cellular technologies is Machine-Type Communications (MTC) [2]. MTC refers to the type of traffic generated by (or directed to) a number of connected machines (such as remote sensors, vending machines, vehicles, and so on) in a given cell, and the possible enhancements in coverage and capacity required for the support of large numbers of such devices. MTC requirements are addressed by 3GPP since Release 8, focusing on GSM and UMTS [3], but LTE has also paid attention to MTC in Release 10 and Release 11 [4] [5] [6]. MTC traffic is characterized by having very low bit rate requirements (of the order of few hundreds of bps), high latencies (of the order of minutes), and a possibly large number of almost inactive devices camped in a single cell.
In order to turn LTE into a cost-effective solution for MTC, studies on how to improve the radio access network procedures for MTC devices are performed in Release 11. Release 12 foresees several mechanisms to extend the coverage and reduce device cost [7], and additional studies are also conducted in order to reduce layer-3 signaling overhead, as well as to increase battery saving through delayed transmissions and the introduction of extended Discontinuous Reception (DRX) cycles [11].
The limiting link in terms of coverage for wireless mobile communication systems is usually the uplink due to limited device transmit power. In order to minimize uplink coverage issues, LTE employs a modified flavour of Orthogonal Frequency Division Multiplexing (OFDM), namely Single-Carrier Frequency Division Multiple Access (SC-FDMA), also known as Discrete Fourier Transform-Spread-Orthogonal Frequency Division Multiplexing (DFT-S-OFDM). SC-FDMA benefits from reduced peak-to-average power ratio (PAPR), therefore power amplifiers are allowed to work with higher efficiency than in OFDM, which in turn prolongs battery life. However proper solutions are still required for MTC devices in poor coverage conditions, as dramatic improvements would be needed for devices with very low transmit power.
In addition, MTC devices are low-cost, low-power devices with limited processing capabilities. One of the most critical issues in OFDM and SC-FDMA comes from the large frequency stability required by the transmissions, which in turn precludes the use of poor local oscillators at the device.
Solutions for coverage enhancements do exist for MTC, most of them consisting in the application of a fixed pattern of packet retransmissions for increased reliability [9]. The drawback of this approach is the increased resource occupation time required for a given information block, as the number of retransmissions is fixed (and usually high) in order to avoid dynamic acknowledgements from the base station. In addition, it does not support large frequency offsets and thus precludes the use of inexpensive local oscillators for MTC.
Other alternatives for coverage extension include reduction of the scheduled bandwidth to very few subcarriers, lower than one resource block (RB), therefore increasing the Signal to Noise Ratio (SNR) [10]. The drawback of this approach is that it very much complicates uplink scheduling in the sub-RB scale, while not introducing any protection for large frequency offsets as the subcarrier spacing would be unchanged.
There are also other proposals disclosing low rate coding or repetition coding, such as document [10] or patent application US-A1-2010/0239046, which are essentially similar in performance to increasing the number of retransmissions.
More specific solutions for enhancing uplink coverage and frequency offset robustness are therefore needed in order to enable the deployment of massive numbers of MTC devices employing SC-FDMA modulation.